The joining of metal components plays a critical role in modern manufacturing and industrial processes. Of the different forms of metal joining, arc welding is widely used to produce a variety of high quality bonds. This is because for many applications, arc welding has proven to be the most cost effective manner of metal joining. However, conventional methods of arc welding suffer from several drawbacks, the most prevalent of which is the inability to penetrate relatively thick workpieces. As a result, such thick workpieces often require multiple passes, which increases welding time, and/or beveling of the edges of the weld seam, which increases preparation time.
Of course, improving penetration reduces the number of passes and the workpiece preparation costs, both of which are important in increasing manufacturing efficiency. In this regard, gas tungsten arc welding (GTAW) (also commonly referred to as tungsten-inert gas (TIG) welding) and the related plasma arc welding (PAW) have gained widespread acceptance as providing greater arc penetration than conventional arc welding methods.
In conventional gas tungsten arc welding, a current is supplied to a non-consumable tungsten or tungsten alloy electrode to form an arc with a workpiece. A special nozzle provides a gas to shield the arc and the weld pool from contamination. The weld bead may be purely autogenous or a filler metal compatible with the base metal may be added, depending on the requirements of the particular operation. While GTAW results in quality welds, like conventional arc welding, the unconstricted arc prevents deep penetration due to the current dissipation.
Plasma arc welding involves the use of a non-consumable electrode which may also be comprised of tungsten or tungsten alloys. However, in addition to a shielding gas, a specialized PAW nozzle is provided which feeds a plasma gas into a chamber surrounding the electrode. Current supplied to the electrode forms an arc with the workpiece(s) adjacent to the weld seam. Heat created by the electric arc ionizes the plasma gas to form electrically conductive plasma, which issues forth from the narrow orifice in the nozzle and carries the arc to the workpiece. This narrow orifice constricts the flow of the plasma gas towards the workpiece into a plasma jet. The arc and plasma jet, collectively referred to as the "plasma arc," generally reach temperatures in excess of 30,000.degree. F. As should be appreciated, the combined constricted arc and high temperature serve to provide greater penetration of the workpiece(s), improved directional stability, and a smaller heat-affected zone than possible with GTAW and other arc welding processes.
In operation, as the PAW torch traverses the weld seam, the heat of the plasma arc melts the base metal and forms an autogenous weld pool. Depending on the thickness of the workpiece(s) being joined and the welding parameters utilized (i.e. the selected arc voltage and current), the extension of the plasma arc may create a small hole extending entirely through the workpiece(s). This hole is termed the "keyhole" and is known to provide several beneficial characteristics, such as reducing porosity and transverse distortion in the bead. As the torch travels along the surface of the workpiece(s) above the weld seam, the keyhole is continuously created and filled. More specifically, the base metal melts upon encountering the plasma arc or the plasma jet and flows from the front of the arc, to the sides, and around to the rear where solidification occurs. Of course, the solidified metal forms the weld bead which is essential to the metal joining operation.
PAW generally provides the deepest penetration of all known arc welding methods, but even with the improved characteristics described above, limitations remain. Even with the constricted arc provided by the PAW torch and the benefits provided by the unique keyhole, proper welding of thicker workpieces remains a problem. More particularly, it is known that during the PAW process, the current required to form the arc generally disperses laterally along the surface of the workpiece(s). As a result of this dispersion, the arc does not extend into the keyhole due to the absence of current. The plasma arc, which is actually the plasma gas jet once it is ionized and heated by the arc, becomes the major heat flux that directly heats the base metal around the keyhole, rather than the arc. However, the plasma jet loses energy as it penetrates into the workpiece, which decreases its penetration capability. This may prevent the keyhole from extending entirely through the workpiece. Furthermore, increasing the current simply increases the weld width and the heat affected zone and, thus, does not significantly improve penetration.
Conventional solutions to overcome these limitations include: (1) machining beveled grooves in the weld seam prior to PAW; or (2) conducting multiple passes with the PAW torch. While both alternatives provide deeper weld penetration, it should be appreciated both require significant additional expense and increase fabrication time. These are factors which are critical in evaluating the comparative efficiency of modern manufacturing processes.
Other non-arc type alternatives for welding thicker workpiece(s) include laser beam and electron welding. These methods permit deeper penetration than both GTAW and PAW. However, the complexity and significant cost of the equipment required renders each method impractical for implementation in many manufacturing applications or for small operations.
Accordingly, a need is identified for an improved method of arc welding for increasing the current penetration through the workpieces, thereby allowing high quality welds to be formed in or on relatively thick workpieces at low current levels. The method would utilize existing welding equipment and, thus, would be inexpensive to operate, as compared to modern types of deep penetration welding, such as laser beam or electron welding. Moreover, the method would be easy to implement and adapt to a wide variety of welding environments.